The invention relates to the field of testing for defects on the surface or surfaces of rotating, magnetic bulk memories of the fixed disk variety, i.e., what are commonly referred to in the computer business as hard disks. More specifically, the invention relates to an apparatus and method for at least doubling the throughput of conventional quality control testing for defects on the surfaces of such hard disks by allowing simultaneous performance of two different kinds of tests whereas in the prior art, the two different kinds of tests had to be performed sequentially.
So called hard disks are comprised of a rigid substrate upon which is coated a thin film of magnetic media. Various types of defects can exist on the surface of the hard disk. Some of these defects relate to defects in the magnetic film while others relate to defects in the flatness of the substrate. These defects are significant since in the process of reading magnetic data stored on the hard disk in the form of magnetic transitions recorded in the thin magnetic film, electrical transitions or pulses are generated and these defects can degrade the amplitude of these pulses and the phase relationship of these pulses to timing windows which are used to read the transitions. That is, binary ones and zeros are recorded in the form of transitions from one magnetic polarity to another magnetic polarity on a portion of the magnetic film called a track. A magnetic read head flies over this track during the read process, and the magnetic transitions on the surface of the film generate tiny magnetic field variations which the read head flies through. When the read head flies through such a transition in the magnetic field, a pulse is generated in the read head. The read circuitry looks for such transitions during timing windows. An amplitude threshold is used in this process. If the pulse so generated does not exceed the amplitude threshold or does not occur within the timing window, the read circuitry will assume that no pulse existed. In fact, what may have happened is that a defect caused the electrical pulse which would normally have been generated as the read head flew over that particular portion of the track to be weak, i.e., low in amplitude, or the defect caused the pulse to occur outside the timing window. In either event, the read circuitry would assume that no pulse and thus no transition occurred at that particular portion on the track thereby resulting in an error in reading the data recorded at that location.
The normal process for testing the magnetic film and substrate for defects is to record a fixed pattern of transitions on each track on each surface of the disk to be tested and then to read these transitions on all the tracks and all the surfaces. This process comprises the first phase of testing. During this first phase, the presence of known transitions at all bit locations on the track are tested for by use of amplitude thresholds and timing windows. If a pulse is found to be "missing" at any particular location by virtue of the pulse representing that location being lower than the amplitude threshold or shifted in phase so as to be outside the timing window, then that location is marked as a defect in a header table. The header table is an area on the magnetic surface which is recorded with information which comprises an index to all the byte locations on the disk which are recordable and all those byte locations on the disk which are not recordable by virtue of there being a defect at one or more of the bit locations which comprise the byte location.
The second phase of quality control testing is then entered. In the second phase of testing, all of the tracks on all of the surfaces are erased. The tracks are then read again. An erased track does not have any recorded transitions, but the signal resulting from reading the track is not a completely flat DC level. There is noise at the output of the read circuitry which results from various sources. Among these sources of noise are the read head amplifiers and various mechanical variations. Among these mechanical variations are the variations in the disk speed, variations in the flatness of the substrate and variations in the repeatability of the head positioning over the particular track being read.
The second phase of testing is accomplished by setting thresholds on either side of the zero signal output level above which the noise level does not rise. Defects will cause what are called "extra pulses" to appear. These extra pulses take the form of pulses which exceed the threshold on either side of the zero signal output level.
Thus, it can be seen that complete quality control testing of a hard disk system using conventional methods and apparatus requires at least one set of revolutions to record all the tracks, one set of revolutions to read all the tracks, one set of revolutions to erase all the tracks and one set of revolutions to read all the erased tracks. Typically, multiple sets of revolutions in each of these phases is performed to detect all defects. This is because not all defects will show up on a single pass of recording and reading or erasing and reading. Therefore, to insure that all defects are detected, multiple passes are used.
Therefore, a need has arisen for a system which can increase the throughput of this testing process. This need is becoming more and more critical as the capacity of disk drives increases steadily with improving technology.